1. Field of the Invention
The present invention relates to a method for the extrusion of polyethylene resins, especially linear low density polyethylene resins and; more particularly, relates to a method of high shear extrusion through the intermediary of which the frequently encountered melt fracture; for instance, such as the sharkskin melt fracture of linear low density polyethylene can be either reduced or even completely eliminated by effecting the extrusion at a high shear rate or shear stress over the lip section of a tubular or sheet extrusion die.
An extremely high percentage of various types of polyethylene resins or polymers, and particularly linear low density polyethylene (LLDPE), is processed or extruded into film material, wherein such film material has found widespread application and acceptance in the packaging industry; for instance, in its employment in the packaging of meat, frozen foods, produce, as boilable or microwavable pouches, and in conjunction with various textile and paper products, industrial liners, shipping sacks, and different types of stretch wrap and shrink wrap sheeting material.
In addition to the foregoing, a considerable volume of extruded polyethylene film material is widely employed in the construction industry and in agriculture, particularly in the form of wide heavy-gauge film webs; attesting to the high level of economic acceptance and importance of this product.
Polyethylene film material of varying kinds, and to an appreciable extent, linear low density polyethylene film, is generally produced through well known tubular blown film extrusion methods and apparatus, or in some instances, through the utilization of sheet extrusion dies. Different types of films may range in size from extruded tubes of about 2" in diameter or less, to huge tubular bubbles adapted to form a layflat film of up to 20 ft. in width, which, when slit, will produce film webs of up to about 40 ft. in width.
2. Discussion of the Prior Art
Among various technical and production problems which are encountered during the extruding of linear low density polyethylene resins, is that of the formation of irregularities in the film material which adversely affect the film properties; for instance, sharkskin melt fractures, whereby the molten polymer extrudates which are propagates through the extrusion dies exhibit deleterious variations in size and shape, resulting from flow defects. Such distortions in the extrudates which are encountered at or above specific critical shear rate or stress levels is associated with resultant melt fracture, particular sharkskin melt fracture. Experimentation has evidenced that melt fracture and an unstable flow of extruded polyethylene resins, including linear low density polyethylene, generally occurs in the extrusion die or upon emerging from the lip or die orifice, and which is commonly known as land-fracture.
Linear low density polyethylene resins without long chain branching generally evidence a narrower molecular weight distribution, which basically imparts two rheological characteristics to the polyethylene resin; in essence, (a) a high shear viscosity; in effect, less shear thinning; and (b) low melt tension; in essence, a low melt strength and a low elongational viscosity. The first-mentioned rheological characteristic results in an encountering of the annoyance of melt fracture, particularly sharkskin melt fracture, during the process of extruding the polyethylene material. Generally, it is possible to effect a reduction in the melt fracture of linear low density polyethylene encountered during extrusion into a film web by decreasing the shear rate or shear stress; by raising the processing temperature, or by increasing the die flow cross-section of the extrusion apparatus; or finally, by decreasing the output rate of the extruder.
In essence, the concept of raising the process or extrusion temperature in order to ameliorate problems which are encountered due to melt fracture, and particularly sharkskin melt fracture of polyethylene materials, such as linear low density polyethylene, in turn, generates the problem of an adverse effect on the bubble stability of linear low density polyethylene for a blown tubular film extrusion process, inasmuch as the polyethylene possesses a low melt strength. Conversely, decreasing the output rate of the extruder in order to obviate melt fracture, causes the overall extrusion operation to become less economical.
Thus, Kurtz, et al. U.S. Pat. No. 4,282,177 discloses a method for reducing sharkskin melt fracture during the extrusion of ethylene polymers, including linear low density polyethylene, in which an attempt is conducted for solving the problem of reducing melt fracture through a change in the cross-section of the die lip or land of the extrusion apparatus, by increasing processing temperature, or alternatively, by decreasing the rate of output of the extrusion apparatus. Although all of these specific paths of approach to the problem provide some improvement in ameliorating the effects and presence of melt fracture encountered during the extrusion of polyethylene resins, they are still inadequate to meet the demands of industry.
The problems of extrudate distortion of linear low density polyethylene resins and similar types of polymers have been discussed to some extent by J. P. Tordella, Trans. Soc. Rheol., 1. 203 (1957); by J. Vlachopulos and T. W. Chan, Journal of the Applied Polymer Science, 21, 1177 (1977); and by T. K. Su, E. Colombo, N. Youngjohn, "Melt Fracture and Flow Instability of Polyethylene Melts", Technical Report T-312 (1983) Mobil Chemical Company. All of the foregoing publications, in varying degrees, discuss the traditional approaches to and the attendant shortcomings in eliminating melt fracture in extruded polyethylene resins; and particularly reducing and/or eliminating sharkskin melt fracture in linear low density polyethylene resins.
Furthermore, Cogswell U.S. Pat. No. 3,920,782 discloses the appearance of sharkskin melt fracture during the extrusion of polymeric materials, and which can be controlled or even eliminated by cooling an outer layer of the material to a reduced temperature level, while maintaining the bulk of the melt at the optimum working temperature. This requires an extensive and complex method of extrusion which, however, does not necessarily produce the desired results in eliminating sharkskin melt fracture of polyethylene extrudates.
Kurtz, et al. U.S. Pat. No. 4,267,146, and Kurtz U.S. Pat. Nos. 4,348,349 and 4,360,494 also disclose other methods employed in reducing melt fracture encountered in various types of ethylene copolymers by varying the configurations of the die orifices and lip surfaces of extrusion apparatus.